Now that the first round
of sequencing is complete and the results are in we can look at what
we have. "This is basically a parts list," explains Eric Lander, Director
of the Whitehead Institute Center for Genome Research in Boston, "it's
a parts list with a lot of parts. If you take an airplane, a Boeing
777, in one sense you'd know a lot. You'd know 100,000 components that
have got to be there, screws and wires and the rudders and things like
that. On the other hand, I bet you wouldn't know how to put it together.
And I bet you wouldn't know why it flies. but you'd be crazy not to
start with a parts list".

Genome
size: We
have 3 billion base pairs of DNAOur parts list
goes a little something like this . . . There are 3,164.7 million base
pairs of DNA in the human genome. To put this in perspective, the
fruit fly genome has about 180 million base pairs, and yeast have 12
million. But the sheer size of an organism's genome does not necessarily
indicate how complex it is.There are 670 billion base pairs in the genome
of an aoemeba, we ourselves only have roughly the same number as a mouse.
That's all very nice, but it's really not the size of the genome that's
important, it's how you use it. That's where genes come in.

Gene number:
We
have fewer genes than expected Genes are
the functional regions of the genome. They can be identified in a sequence
because they are usually preceded and followed by recognizable 'start'
and 'stop' sequences that can be identified by a computer. Both Celera
and HGP agree that humans have somewhere upwards of 35,000 genes. Scientists
had originally expected more than 100,000 genes in the human genome
to account for our higher complexity. Yet although we do have one of
the higher gene counts, we only have twice as many genes as a fruit
fly or worm, and about the same number as a mouse.

The
similarities of the human gene count to other creatures proved
a real shock for scientists. Pic courtesy: DOE
Human Genome Program

So, how is it that humans
can have the same number of genes as a mouse, and that 50% of those
genes are identical to those in yeast? After all, we are humans - we
have schools, doctors, designer clothing, cities and software conglomerates.
We even figured out relativity. Surely we must have more or different
or better genes than anything else? The truth is, we have a lot of the
same genes as many other organisms because our cells and their cells
function in much the same way. The machinery that allows a cell to grow,
divide, store energy, and do the other basics of life work the same
way in us as they do in yeast. "The fundamental mechanisms of life were
worked out only once on this planet and have gotten reused in every
organism on the planet," says Eric Lander, "Evolution doesn't go and
reinvent something when it doesn't have to".

Busy,
busy: We do more with our genesIf you are feeling
inadequate don't worry, you really are special. Our complexity as a
species has a lot to do with the fact that we do more with the genes
we have. Imagine a gene that has 4 segments: A-B-C-D. Many organisms
may just make a protein from A-B-C-D, in
that order. But if the machinery that made the protein were to skip
segments, you could also make the proteins A-B-D, B-C-D, A-C-D, A-D,
and so on. It's called 'alternative splicing' and it appears that the
more complex the organism, the more this method is used. You can start
to see how our full set of proteins, also called our "proteome", could
get pretty complex after a while. The 35,000 genes that were identified,
could reasonably give us upwards of 100,000 different proteins.

Junk-DNA:
Genes make up only 2% of the human genomeWe have
3.2 billion nucleotides but only 35,000
genes - the sums don't seem to add up. How can there be such a discrepancy
between genome size and gene numbers? 98% of the human genome is non-coding
"junk" DNA. Most of it is long stretches of repeating sequences that
don't appear to have any direct function at all. A comparison with the
genomes of other organisms reveals that the amount of "junk" DNA, not
the gene count, is the main reason for a difference in genome sizes.
Still, "junk" is such a harsh term, and in reality junk-DNA may not
be entirely useless, in fact it may actually have some role in the way
our genes are separated.

Bob Waterson, Director of
the Genome Center at Washington University in the U.S. explains, "the
distribution of genes on mammalian chromosomes
is uneven, making for a striking appearance . . . in some regions, genes
are crowded together much like buildings in urban centers. In other
areas, genes are spread over the vast expanses like farmhouses on the
prairie. And then there are large tracts of desert, where only non-coding
'junk DNA' can be found. Each region tells a unique story about the
history of our species and what makes us tick." The big repeats tend
to be in the deserts. This serves to make the gene cities more separate
from one another. For all we know, this may have an affect on how each
gene city is used.

For the most part though,
these big regions of junk-DNA seem to have just hitched a ride on our
genome over the last 4 billion years or so, and can tell us a little
about the history of our genome. Occasionally, organisms will do a little
house-cleaning and toss out much of the junk DNA. The reason we have
more junk-DNA than a fruit fly is that the fruit fly cleaned house about
12 million years ago. and it seems we last took out the garbage 800
million years ago. Both sequencing projects agree that around 50 million
years ago, we seemed to have stopped collecting junk. Mice, on the other
hand continue to collect it.

You
are unique . . . just like everybody elseOur gene
pattern makes us human, and slight differences in that pattern make
each human unique. Your genetic code is 99.9 percent identical to that
of the next person you meet, whether you like them or not. A difference
of one letter in the gene code is called a "Single Nucleotide
Polymorphism", or SNP for short and your collection of SNPs help to
make you an individual. But in the wrong place, SNP's can be disastrous.
Diseases like sickle cell anaemia, cystic fibrosis, tay-sachs disease,
and many cancers are caused by the changing of a single nucleotide in
3 billion. Recent evidence suggests that our individual SNP patterns
may be responsible for the way we react to drugs.

This is where having a complete
human genome sequence is already paying off. Human genome researchers
are in the process of making a catalogue of all the known human SNPs
and their location on the human genome. Right now 1.4 million SNPs have
been mapped, and already new disease genes have been identified, saving
years of individual research. Not only that, but mapping human diversity
is giving some scientists an idea of our ancestry. Using the SNP map,
they have been able to estimate that all the humans on this planet have
evolved from a small population of 10,000 to 50,000 people who lived
around 100,000 years ago.